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Compressor Issues

I recently serviced a unit that is about 25 years old ( I know , people think they should run for life ) and the problem was warm air from vents so I start trouble shooting, finding that the unit was severly low on freon. I evac.. and pump up with nitrogen to find a leak and did find a very small leak and fixed it.
So I Vacumm unit to 375 microns and let it stand and everything is good.
Weighed in the charge and started unit, checked superheat and it was great. Then I check Temp change acorss evep. coil and it was text book, So I left the owner with a great running unit.

Well today I get a call that the unit is blowing warm air so I go to check it out and when i got there it was severly low on freon ( Now i'm stratching my head for real) So I pump it up with nitrogen and bubble everything and I mean everything. Absolutely no leaks , No bubbles , Pressure remains Same for long period of time. I'm thinking If the unit is going to leak 3.5 pounds of freon in four days it's a substantial leak. but I can not find anything at all. My thought is maybe the compressor is leaking internally, But i'm not that familiar with compressors yet and i'm looking for some advice and the good news is that I finally got the homeowner to buy a new unit but i'm still wondering about the old one for general knowledge.

I recently serviced a unit that is about 25 years old ( I know , people think they should run for life ) and the problem was warm air from vents so I start trouble shooting, finding that the unit was severly low on freon. I evac.. and pump up with nitrogen to find a leak and did find a very small leak and fixed it.
So I Vacumm unit to 375 microns and let it stand and everything is good.
Weighed in the charge and started unit, checked superheat and it was great. Then I check Temp change acorss evep. coil and it was text book, So I left the owner with a great running unit.

Well today I get a call that the unit is blowing warm air so I go to check it out and when i got there it was severly low on freon ( Now i'm stratching my head for real) So I pump it up with nitrogen and bubble everything and I mean everything. Absolutely no leaks , No bubbles , Pressure remains Same for long period of time. I'm thinking If the unit is going to leak 3.5 pounds of freon in four days it's a substantial leak. but I can not find anything at all. My thought is maybe the compressor is leaking internally, But i'm not that familiar with compressors yet and i'm looking for some advice and the good news is that I finally got the homeowner to buy a new unit but i'm still wondering about the old one for general knowledge.

Any suggestions?

Thanks,

my opinion..you need a electronic leak detector,easy to find leaks in coils,and what do you mean compressor leaking internally???if it is hermetic??,you would find the leak with a elec..if it is leaking internally,by means of the valves,you would see a high to low side leak..if thats what you mean..
its hard to find a leak in a ciol without an elec,but over 3 pounds of refrigerant in four days,you would think you would hear somthing if you pump it up over 150 pound with nitro

I told the home owner that they needed a new unit but if youv'e heard it once youv'e heard it a thousand times ( I'm just trying to get by this summer and next summer I will replace it, lol ). If they would rather spend their money trying to fix this dinosuar I'm not going to turn it down.

I did pressurize and disc.. the guages and then check for leaks and found nothing my next move is a leak detector